It is well known to manufacture paper by a process that comprises flocculating a cellulosic thin stock by the addition of polymeric retention aid and then draining the flocculated suspension through a moving screen (often referred to as a machine wire) and then forming a wet sheet, which is then dried.
In order to increase output of paper many modern paper making machines operate at higher speeds. As a consequence of increased machine speeds a great deal of emphasis has been placed on drainage and retention systems that provide increased drainage. However, it is known that increasing the molecular weight of a polymeric retention aid which is added immediately prior to drainage will tend to increase the rate of drainage but damage formation. It is difficult to obtain the optimum balance of retention, drainage, drying and formation by adding a single polymeric retention aid and it is therefore common practice to add two separate materials in sequence.
EP-A-235893 provides a process wherein a water soluble substantially linear cationic polymer is applied to the paper making stock prior to a shear stage and then reflocculating by introducing bentonite after that shear stage. This process provides enhanced drainage and also good formation and retention. This process which is commercialized by BASF under the Hydrocol® (trade mark) has proved successful for more than two decades.
This Hydrocol® (trade mark) system of making paper is a very efficient microparticle system for a wide range of paper grades including liner board and folding box board production. The benefits of this system include high retention levels, good drainage, good formation, good machine cleanliness, good runnability and a cost efficient system.
Subsequently, various attempts have been made to provide variations on this theme by making minor modifications to one or more of the components.
EP-A-335575 describe such a process in which a main polymer selected from cationic starch and high molecular weight water-soluble cationic polymer is added to a cellulosic suspension after which the suspension is passed through one or more shear stages followed by the addition of inorganic material selected from bentonite and colloidal silica. In this system a low molecular weight cationic polymer is added into the suspension before the addition of the main polymer. It is indicated that the low molecular weight polymer usually has a molecular weight below 500,000 and usually above 50,000, often above 100,000. Suggested low molecular weight cationic polymers include polyethyleneimine, polyamines, polymers of dicyandiamidesformaldehyde, polymers and copolymers of diallyl dimethyl ammonium chloride, of dialkyl amino alkyl(meth)acrylates and of dialkyl amino alkyl(meth)acrylamides (both generally as acid addition or quaternary ammonium salts). The process was said to improve processes in which there is a high amount of pitch or processes with a high cationic demand.
A further development of this type of process was subsequently disclosed in EP-A-910701 in which two different water-soluble cationic polymers or added in succession to pulps followed by subjecting the pulps to at least one shearing stage followed by the addition of bentonite, colloidal silica or clay. Specifically polyethyleneimines having a molar mass of more than 500,000 or polymers containing vinyl amine groups having a molar mass of between 5000 and 3 million are added to the pulp and then high molecular weight cationic polyacrylamides.
EP-A-752496 discloses a papermaking process in which a low molecular weight cationic polymer having a molecular weight below 700,000 and a cationic and/or amphoteric high molecular weight polymer are added simultaneously to the thin stock with anionic inorganic particles such as silica or bentonite being dosed into the thin stock suspension. The low molecular weight cationic polymer includes polyethyleneimine and polyvinyl amine. The polymers are generally added separately although it is indicated that the two cationic polymers can be added as a mixture. It is also indicated that the polymers can be added before a shear stage although the exact addition points are not indicated. It is stated that this process results in improved drainage and/or retention compare to processes in which the high molecular weight cationic or amphoteric polymer is used alone in conjunction with anionic inorganic particles.
U.S. Pat. No. 6,103,065 discloses a papermaking process involving the addition to a paper stock after the last point of high shear at least one high charge density cationic polymer of molecular weight between 100,000 and 2 million with a charge density in excess of 4 mEq per gram and either concurrently or subsequently adding at least one polymer having a molecular weight more than 2 million with a charge density below 4 mEq per gram. Subsequent to the two polymers a swellable bentonite clay is added to the stock. The high charge density polymer can be polyethyleneimine homopolymers or copolymers or polymers produced from vinyl amines. This document indicate that the process improves conventional bentonite programs by employing less polymer and improving press section dewatering which increases the solids entering the dryers thereby reducing the drying requirements.
In the production of paper and paperboard the paper machine can become limited by the amount of water retained in the final web after the press section when the paper machine is using maximum drying energy. The retention of fibre and filler articles is also limited when using standard retention and drainage aid systems due to potential paper quality issues. The retention and dewatering performance can be improved by using higher additions are retention and drainage aid chemicals such as polyacrylamide and bentonite. However, larger doses of these chemicals can negatively impact on the physical properties of the paper sheet.
A particular disadvantage of many conventional microparticle systems is that drainage tends to increase simultaneously with increasing retention. Although this may have been perceived as an advantage several years ago, with modern high-speed paper machines very high drainage can be a disadvantage. This can be the case for gap former machines and multi-ply fourdrinier machines. Folding box board is normally produced on multi-ply fourdrinier machines in which the major ply is the middle layer (typically about 150 to 400 g/m2). The requirements for these grades are good retention for the lower basis weight and good drainage for the high basis weight. Nevertheless in most cases it is necessary to reduce the paper machine speed for the higher basis weight sheets because of these drainage limitations. In many cases simply increasing the retention aid components the drainage on the wire can be improved but the water release in the press tends to be reduced. Further, formation can also be adversely affected.
It would be desirable to provide an improved process for making paper and board. Furthermore, it would be desirable to overcome the aforementioned disadvantages.